Until recently the rotational speed required in rotary devices was at most been about 100,000 rpm and laminated electrical steel sheet was used for the material of rotor cores. Recently, superhigh speed rotation as high as 200,000 or 300,000 rpm has been demanded and thus the centrifugal force applied to the rotor could exceed the strength of the electrical steel sheet. Further, inner magnet type motors are increasingly used and the load applied to the material of the rotor during rotation becomes large. Thus, the fatigue strength of the material is becoming important in many cases.
In other cases, the contact surface of electromagnetic switch is worn during its use, and thus magnetic materials superior in not only electromagnetic properties, but also wear resistance are desired for the use.
To meet with these needs, recently high strength non-oriented electrical steel sheet has been studied. Several proposals have been made. For example, Japanese Published Patent Application No. 1-162748 and Japanese Published Patent Application No. 61-84360 propose a material using a slab increased in Si content and further containing one or more of Mn, Ni, Mo, Cr, and other solid solution strengthening components, but the sheet is liable to break easily in rolling and causes less productivity and less yield. Thus the sheet has a room for improvement. Furthermore, since Ni, Mo or Cr are included in large amounts in the steel, the material becomes extremely expensive.
Japanese Published Patent Application No. 61-87848 discloses producing high strength non-oriented electrical steel sheet by rapid solidification from a melt containing 2.5% or more of Si. Japanese Published Patent Application No. 8-41601 discloses improving the rollability by wrapping a high Si steel containing 2.5% or more Si by low Si steel containing 2.0% or less Si. Since these proposals use special processes, the sheets cannot be produced by the production facilities for conventional electrical steel sheet and therefore are difficult to be produced industrially.
With the above methods utilizing solid solute strengthening by solute elements, from the view point of magnetic properties, the saturation magnetic flux density of the material is inherently low, and thus the magnetic flux density of the product sheet is inevitably low. Further, from the view point of crystal structure, the methods inherently refines grain size, so while these are preferable in terms of increasing the strength, there is the problem that the core loss ends up rising.
Further, to strengthen a material, utilizing precipitates may also be considered, but precipitates also end up degrading the magnetic properties from the viewpoint of the magnetic flux density and core loss due to the effects of the precipitates themselves and the refining the crystal structure. In this way, high strength electrical steel sheets have inherent problems wherein the magnetic properties originally required are remarkably degraded.
In particular, with materials strengthened by refining grain size or by precipitates, when punched to an article for electrical appliances such as motors etc., in the stress relief annealing (SRA) process for relieving the fabrication stress introduced to the steel sheets, growth of the crystal structure or precipitates occurring while holding the steel at a high temperature is unavoidable and therefore the strength is decreased. Further, use of high strength materials accelerates the wear of the dies when punching the steel into parts for electrical appliances, in particular in the shearing process, so becomes a cause of raising the cost of production of the electrical appliances.